Lithium ion secondary batteries have a separator between the positive electrode and the negative electrode, the separator having the functions of electrically insulating these electrode plates and further holding an electrolyte. The separator for lithium ion secondary batteries is currently made of a microporous thin film sheet composed mainly of polyethylene. When such a thin film sheet is used, an internal short-circuit may occur, since an active material that has fallen off from electrode plates in a manufacturing process of a battery or foreign matter included in the battery may penetrate the thin film sheet.
In order to prevent the occurrence of internal short-circuits in lithium ion secondary batteries, and as an effective means for preventing the expansion of short-circuiting, there has been a proposal to form a protective film (porous insulating layer) containing insulating particles (inorganic oxide filler), such as alumina powder, and a resin binder on the surface of the positive electrode or the negative electrode (Japanese Patent Publication No. 3371301). The porous insulating layer is usually formed by applying a slurry containing an inorganic oxide filler and a resin binder on the electrode surface and drying it.
Generally, the electrode for lithium ion secondary batteries is formed of an electrode plate in sheet form. Such an electrode sheet comprises an electrode core member and an electrode material mixture carried on each side of the electrode core member. The electrode sheet needs to have a lead-forming part for connecting a current-collecting lead. As the lead-forming part, an exposed part of the electrode core member (current collector), i.e., a part having no electrode material mixture applied thereon, is usually utilized. It is common from the structural requirements of the battery that there is a location displacement between the part having no electrode material mixture applied thereon on one side of the electrode core member and the part having no electrode material mixture applied thereon on the other side thereof.
A typical method for applying an electrode material mixture to an electrode core member is a method of intermittently applying a paste containing an electrode material mixture to an electrode core member using a die coater (Japanese Patent Publication No. 2842347). There has also been proposed a method of applying a paste containing an electrode material mixture to an electrode core member by means of a gravure roll without forming a part having no electrode material mixture applied thereon (Japanese Laid-Open Patent Publication No. 2001-179151). However, this method requires peeling a part of the electrode material mixture off the core member in order to form a part having no electrode material mixture applied thereon. There has also been proposed a method in which a part of an electrode core member is masked with tape, a paste containing an electrode material mixture is applied to the electrode core member, and thereafter the tape is peeled off. Further, there has also been proposed a method in which a part having no electrode material mixture applied thereon is formed by clotting a part of an electrode material mixture applied to an electrode core member and then peeling it off (Japanese Laid-Open Patent Publication No. Hei 10-247490).